As input/output (I/O) speed and the total number of I/Os required for high performance semiconductor chips have increased dramatically, the need for increased numbers of interconnect lines with low line impedance variation in chip packages has increased as well. To address this need, manufacturers have used multi-layered packages where several layers of conductors are separated by layers of dielectric material.
In printed circuit board (PCB) and integrated circuit (IC) manufacture, often silicon dies are to be connected to a mother board. This connection of a die to a mother board is known as a package. The die may be flip mounted to a piece of substrate called an Organic Land Grid Array (OLGA). The OLGA is typically formed of a core of FR4 material used commonly in the manufacture of printed circuit boards.
On two sides of the OLGA board are typically a series of built-up layers, formed from alternating layers of dielectric material and conductive material. Patterns may be built in the metal or conductive layer through various etching processes such as wet etching which are known in the art and will not be described further herein. Plated through holes called vias are used to make interconnects between various layers of metal. Using these layers and vias, several layers of interconnections may be built up.
In an OLGA packaging technology, input/output functions are accomplished using metal traces between the layers. These traces are typically grouped. Each trace has an impedance generated by its geometry and location on the OLGA. Due to the manufacturing technology and material requirements, OLGA packages require a number of degassing holes to be formed in the metal layers to allow for proper operation. Degassing holes allow gas to be evaporated so that bubbles do not form in the package.
Traces may be routed over or under the degassing holes, or around the degassing holes, or a combination thereof. Since the traces are not in the same location on the OLGA, the traces have an impedance variation, or mismatch. OLGA trace impedance variation arises from two separate origins, manufacture variation and design variation. Manufacture variation and design variation add statistically to yield overall impedance variation, or mismatch.
Manufacture variation arises from geometry variations of traces, including trace width, trace thickness, dielectric thickness, and variation of the dielectric constant of a dielectric. Design variation is introduced from package design. When traces are run in an OLGA, they have a routing direction and a fan-out direction. Traces must be routed from the die to the package. When the traces are routed, the direction of the trace is referred to as the routing direction. The fan-out direction is typically 45 degrees from the routing direction, either plus 45 degrees or minus 45 degrees.
A typical degassing hole pattern has a grid-like array of degassing holes aligned vertically between two layers, as is shown in FIG. 1. In FIG. 1, the degassing holes 102 of the top and bottom layers are exactly aligned in the x and y directions. When traces such as trace 1 and trace 2 are used with a degassing hole alignment scheme as shown in FIG. 1, trace 1 has less metal from the conductive layers both above and below the trace than trace 2. The difference in the amount of metal above and below traces 1 and 2 continues when the traces are run in the fan-out direction. The degassing hole pattern of FIG. 1 leads to design impedance variations alone being on the order of 20%.
Another degassing hole pattern shown in FIG. 2 has another grid-like array of degassing holes 202 staggered from the degassing holes 204 of the next layer. In FIG. 2, the degassing holes are staggered in the x direction between layers. The distance between degassing holes is known as pitch. The degassing holes of the layers in FIG. 2 are staggered by a half pitch in the x direction. When traces such as trace 3 and trace 4 are used with a degassing hole alignment scheme as shown in FIG. 2, trace 3 has less metal from the conductive layers both above and below the trace than trace 4. The variation in the amount of metal above and below the traces is lowered in fan-out at a 45 degree angle from the x direction because of the staggering of the holes. Still, significant design impedance variations are present with the degassing hole pattern of FIG. 2.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a degassing hole pattern that reduces design variation in an OLGA package.